Acid Oceans Are Stripping Sharks of Their Scales

Sharks are covered in denticles (tiny, tooth-like scales), which provide the animal with a tough, protective layer—but are also highly vulnerable to ocean acidification.

Consistent exposure to a low pH can degrade the denticles, essentially stripping sharks of their scales and leading to problems with their ability to swim and catch prey. This is a situation that could worsen as climate change turns the oceans increasingly acidic, say scientists writing in Scientific Reports.

A team of researchers led by Luntz Auerswald, a professor in the Department of Environment, Forestry and Fisheries at Stellenbosch University in South Africa, investigated the effects of long-term exposure to acidified water for a period of nine weeks.

After nine weeks of exposure, the team found that around a quarter of denticles were damaged. In comparison, 9.2 percent of denticles were damaged in a control group of three sharks held in non-acidic water.

This, the authors conclude, suggests the corrosion may weaken the protective quality of the shark's skin and reduce the ability to swim in open-water species, such as the more famous great white. The reason for the latter is that denticles decrease drag and turbulence—in turn, increasing speed and allowing the individual shark to swim more quietly, so that they can better sneak up on prey.

In a separate experiment, the researchers took blood samples from 36 sharks exposed to acidified seawater for various stretches of time, finding that concentrations of carbonate increased as concentrations of carbon dioxide in the blood increased. It suggests the individual sharks are able to acclimatize to bursts of high carbon dioxide levels, as the carbonate stops the blood from becoming too acidic.

"From the changeable environment the shysharks come from (in terms pH), we expected that they would be able to regulate their acid-base balance in the short term as a response to a lowered pH," Auerswald told Newsweek. "We were unsure, but not surprised, that they can keep this regulation up for extended periods. The corrosion of the denticles, however, came as a surprise. We did not expect this."

Puffadder shyshark
Researchers tested to see how long-term exposure to acidic water affected the denticles of puffadder shysharks like this one lurking in the coral. Madelein_Wolf/iStock

The diminutive puffadder shyshark, found in the waters along Africa's southern tip, was chosen as a proxy to draw conclusions about ocean acidification's effect on other shark species—including the great white and the hammerhead.

"This species is small, easy to rear and handle, easy to catch and not endangered or protected," said Auerswald.

While the experiments were conducted on a relatively small number of individuals—only three sharks were exposed to nine weeks of acidified water—the research implies increasing levels of ocean acidification (spurred by carbon dioxide absorption) will have an increasingly detrimental effect on denticle-carrying marine organisms, like sharks and rays.

The damage caused by acidification not only hinders the protective element of a shark's skin but certain species' ability to swim. The researchers also suspect a similar level of corrosion could damage sharks' teeth, making it harder to feed.

"There are of course many other processes that could be compromised such as reproduction," Auerswald added. "However, we can only speculate about this at the moment."

"This study pertinently shows a case of another marine animal being threatened by ocean acidification," Jean-Luc Solandt, Principle Specialist in Marine Protected Areas at the Marine Conservation Society, U.K., told Newsweek. "But more widely, it is likely to have a greater impact on ocean system processes leading to less productive seas. If this is happening to the skin of a shark, think of the massive impact on plankton, the base of the food web and a massive absorbing agent of CO2."

Auerswald and his team are continuing to investigate the effects of ocean acidification on different marine organisms, including rock lobsters, abalone and sea urchins. Meanwhile, they plan to carry on experimenting on shark teeth and denticles into 2020 to back up their findings.

"We hope that people realise what impact ocean acidification has on marine organisms," said Auerswald. "The focus is mostly on ocean warming whereas acidification is hardly in the mind of the public but also decision makers."